1. Field of the Invention
This invention relates to an optical apparatus having a lens driving device.
2. Description of the Related Art
The camera apparatuses of the kind continuously taking a motion picture such as video cameras or the like have been arranged to drive or stop a focusing lens group or to determine the lens driving direction according to the result of determination of a near, far or in-focus state detected by an automatic focusing (hereinafter referred to as AF) device. In some cases, the speed of driving the lens group is arranged to be variable according to the degree of blur resulting from a defocus state.
FIG. 8 shows by way of example an apparatus comprising in combination an automatic focusing device which is of the kind determining an in-focus state when a high-frequency component of a luminance signal coming from an image sensor 101 reaches its peak and a so-called rear focus zoom lens in which a group of lenses other than a first lens group forms a focusing lens group. In the case of FIG. 8, first and second lens groups 102 and 103 are arranged to be movable in association with each other by a power varying action. A zooming action is performed by moving the first and second lens groups in a predetermined relation. The shortest focal length is obtained when the total lens length becomes a minimum length. The illustration includes an iris 104; a stationary lens group 105; and a movable lens group 106 which has a focusing function and a compensating function which is performed during the process of zooming.
This movable lens group 106 must be arranged to vary its locus according to a distance to a picture-taking object. If the zoom lens system is fixed to a single focal length, a focusing action is performed by drawing out or in the lens group 106. The lens system is focused on an object located at the nearest distance with the lens group 106 drawn out. The illustration further includes a CPU 107; boundary data 108 provided for defining areas which will be described later; and speed data 109 and rotating direction data 110 stored for the speed and direction of each of the areas determined by the position of the lens group 106 which can be found from the result of detection made by a zoom encoder reading circuit 111 and the output of a stepping motor driving pulse output circuit 112.
A reference numeral 113 denotes a zoom operation switch. A numeral 114 denotes a main (or power supply) switch. A power-on reset circuit 115 is arranged to reset a step pulse count to zero by setting a stepping motor 117 in a predetermined position when the power is turned on. A stepping motor driver 116 is arranged to drive the stepping motor 117 in response to a driving instruction given by the CPU 107. A zoom motor driver 118 is arranged to drive a zoom motor 119 in response to a driving instruction given by the CPU 107. A modulation actuator driver 123 drives a modulation actuator 120 which is arranged to change the position of an image sensor 101 such as a CCD or the like in the direction of focusing, i.e., in the direction of an optical axis. A piezoelectric actuator, a voice coil or the like can be employed as the modulation actuator 120. Further, an action equivalent to the action of the modulation actuator 120 can be accomplished by amplitude-modulating the lens group 106 in the optical axis direction with the stepping motor 117. An F value reading circuit 121 (an aperture encoder circuit) is arranged to read an aperture value through the output of a detecting element such as a Hall element or the like disposed in a driving meter of the iris 104.
At the AF device 122, the amplitude of the modulation actuator 120 or the stepping motor 117 is determined on the basis of information on the aperture value, because the depth of field varies with the aperture value to vary a relation between the change of the degree of focus and the displacement of the image sensor 101.
The AF device 122 is described with reference to FIG. 9 as follows: The image sensor 101 which is a CCD or the like is arranged to produce an image signal. This signal is amplified by a preamplifier 151. After that, a high-frequency component of the amplified image signal is taken out by means of a high-pass filter (HPF) 152. From the image signal thus output from the high-pass filter 152, a signal which corresponds to the inside of a predetermined distance measuring field is then taken out by means of a gate circuit 153.
The signal which has passed through the gate circuit 153 is subjected to a detection process at a detection circuit 154. The output of the detection circuit 154 is integrated by an integrating circuit 155. The integrated signal is converted into a digital signal by an A/D converter circuit 156. FIG. 10 shows the value FV of the high-frequency component of this signal. As shown, the value FV is large when the image is sharp. The value FV becomes small when the image is not sharp and is blurred. The in-focus state of the lens group 106 can be determined by detecting a position of the lens group 106 where the value FV is obtained at the largest value as shown in FIG. 10. At the CPU 107, the driving direction of the motor 117 and the driving speed of the motor 117 are determined on the basis of the absolute value of the value FV, a difference between the previous and current values of the value FV, and the aperture value obtained from an aperture encoder 157, etc. Again referring to FIG. 8, the position of the lens group 106 is controlled not only according to the result of distance measurement performed by the AF device 122. During the process of zooming, in particular, the lens position is controlled by starting the stepping motor 117 simultaneously with the zoom motor 119 on the basis of the area data stored for every designated area, which is determined by the output of a zoom encoder and the number of pulses of the stepping motor 117.
FIG. 11 is a graph showing the positional relation between the variator lens group and the focusing lens group obtained at various object distances. The position of the variator lens group is shown on the abscissa axis and that of the focusing lens group on the ordinate axis. In FIG. 11, the object distances are indicated in such a way as .infin., 2 m, 0.4 m, and 0.002 m. Assuming that the moving speed of the variator lens group in zooming is unvarying, the abscissa axis can be regarded as time. Assuming that the time required for distance measurement by controlling the lens solely by means of the automatic focusing device during the process of zooming is t1, when the lens is zoomed from an in-focus state obtained at a point P1 in the direction of wide-angle (W), the lens group 106 does not move in response to zooming until the lapse of the time t1. Therefore, the positional relation between the two lens groups becomes as indicated by a point P2 to bring forth a circle of confusion as much as "d1 X (the sensitivity of the lens group 106)/F-number" . If this degree of blur presents a problem, the blur can be greatly improved by simultaneously starting the zoom motor 119 and the stepping motor 117 at a speed obtained by differentiating with the point P1 an object distance locus which includes the point P1. For example, zooming in the wide-angle direction from the point P1 reaches a point P3 after the lapse of time t1. With an ideal point assumed to be a point P4, the blur occurring can be expressed as "d2 X (the sensitivity of the lens group 106)/F-number". This clearly shows the great effect of the above-stated simultaneous start.
Therefore, a map arranged within the CPU 107 is divided, as shown in FIG. 12, both in the direction of the variator lens group positions and the direction of the focusing lens group positions according to the required degree of definition. Then, a representative speed for each of the areas is stored within a memory of an electronic circuit in such a way as to minimize the degree of blur occurring in the process of zooming. During zooming, the speed data stored comes to show a maximum speed value at a point in the neighborhood of a tele-photo end to cause the stepping motor 117 to be rotated at this value.
The speed and position of the stepping motor which is employed as a focusing motor are controlled in the above-stated manner. There are various exciting or current variation methods for driving a stepping motor. Each of these different methods has its own feature. The exciting methods generally employed in driving a two-phase stepping motor are shown in FIGS. 13(a) to 13(d). FIG. 13(a) shows one-phase excitation or current and FIG. 13(b) two-phase excitation or current. These methods permit driving at a higher speed than other methods. FIG. 13(c) shows one/two-phase excitation, which permits stopping position control within one half of the two-phase excitation. FIG. 13(d) shows another exciting method. In this case, a portion of the one/two-phase excitation corresponding to the two-phase excitation is divided stepwise to permit finer control. Hereinafter, this exciting method is referred to as the MS excitation.
The characteristic of the stepping motor can be expressed as shown in FIG. 14 by taking the rotating speed on the abscissa axis and the torque thereon on the of ordinate axis. In FIG. 14, a reference numeral 160 denotes a pull-in torque. The inner side of the pull-in torque 160 is called a self-start area within which the stepping motor can make a self-start. A numeral 161 denotes a pull-out torque. The area between the pull-in torque 160 and the pull-out torque 161 is called a through area. In the through area, the stepping motor is usable by gradually raising the rotating speed from the self-start area. Further, this characteristic varies when the exciting method changes even with the same motor used.
Heretofore, in cases where a stepping motor is used as a focusing motor, the motor is used within the above-stated self-start area in accordance with one of the various exciting methods mentioned.
In the case of the conventional apparatus mentioned by way of example in the foregoing, only one of the exciting methods is employed in driving the stepping motor. As a result of that, the conventional apparatus has had the following shortcomings: (I) the length of time for avoiding a blurred state becomes long when position control is finely performed in the neighborhood of an in-focus point; (II) the control becomes difficult when the stepping motor is used within the through area; (III) if position control is finely performed for AF (automatic focusing), it is difficult to drive the stepping motor at a required rotating speed in zooming; (IV) and further, in order to eliminate the above-stated shortcomings, the size of the stepping motor must be increased.